As new oil and gas wells are coming on line to meet increasing energy demand, many of the existing gas processing facilities are not well adapted to accommodate to the often richer gas compositions from these new wells. Most typically, such gas compositions are rich in NGL (Natural Gas Liquid) and contain substantial quantities of heavier hydrocarbons (e.g., C4 to C6), which frequently creates operating problems when fed to existing NGL recovery units.
For example, many known cryogenic expansion configurations and processes (e.g., as described in U.S. Pat. Nos. 4,157,904 to Campbell et al., 4,251,249 to Gulsby, 4,617,039 to Buck, 4,690,702 to Paradowski et al., 5,275,005 to Campbell et al., 5,799,507 to Wilkinson et al., and 5,890,378 to Rambo et al.) are configured for relatively high NGL recovery, however, only when supplied with a relatively narrow range of gas compositions, such as lean feed gases and/or feed gases with low C5+ content. Consequently, throughput and NGL recovery in such known plants is often reduced when feed gas compositions are significantly different than originally planned, which often translates to significant product revenue loss. In such instances, processing equipment will typically have to be revamped to maintain a high NGL recovery, which often requires extensive shutdown of the plant at substantial product revenue loss. Moreover, significant capital expenditure is necessary, for example, to include new refrigeration units, new heat exchangers, or to re-wheel turboexpanders. In other cases, the demethanizer column must be revamped (e.g., with high capacity trays) or even replaced to handle the richer gas. Alternatively, plant throughput and NGL recoveries can be reduced, which significantly reduces product revenues.
In still other examples (e.g., U.S. Pat. No. 6,182,469 to Campbell et al., U.S. Pat. No. 6,244,070 to Lee et al., and U.S. Pat. No. 5,890,377 to Foglietta), the demethanizer reboilers are closely heat integrated with the feed gas exchangers, and therefore have an increased duty with an increase in richness of the feed gases. In such plants, liquids from the intermediate separators are fed to various tray locations in the demethanizer, which are optimized for the design feed composition. However, the fractionation efficiencies will be significantly reduced when operating on different feed gas compositions. In addition, the absorber overhead is often cooled and refluxed by a lean stream which composition is also dependent on the feed gas composition. It should be noted that high recoveries of the NGL components (C2 to C5 and heavier) in such plants are generally based on an optimum design for a narrow range of gas compositions. Consequently, as feed gases become richer (i.e. higher C4-C6 component content), these plants typically fail to achieve the desirable throughput and recovery due to the limitations of the refrigeration capacity and the demethanizer system that was originally designed for leaner gases.
Therefore, although various configurations and methods are known to recover NGL from a feed gas, all or almost all of them suffer from one or more disadvantages, especially where the feed gas is relatively rich. Therefore, there is still a need to provide methods and configurations for improved NGL recovery.